Get the proper default settings for Firefox and IE

We were using Dartium settings due to what I assume was a copy-paste
error on my part. I don't think this actually affected anything but
tests, since when running tests we never used the default
ExecutableSettings.

lib/src/runner/browser/firefox.dart

@@ -35,7 +35,7 @@ class Firefox extends Browser {
   /// Starts a new instance of Firefox open to the given [url], which may be a
   /// [Uri] or a [String].
   static Future<Process> _startBrowser(url, ExecutableSettings settings) async {
-    settings ??= defaultSettings[TestPlatform.dartium];
+    settings ??= defaultSettings[TestPlatform.firefox];
     var dir = createTempDir();
     new File(p.join(dir, 'prefs.js')).writeAsStringSync(_preferences);
 

lib/src/runner/browser/internet_explorer.dart

@@ -22,7 +22,7 @@ class InternetExplorer extends Browser {
   /// Starts a new instance of Internet Explorer open to the given [url], which
   /// may be a [Uri] or a [String].
   static Future<Process> _startBrowser(url, ExecutableSettings settings) {
-    settings ??= defaultSettings[TestPlatform.dartium];
+    settings ??= defaultSettings[TestPlatform.internetExplorer];
 
     return Process.start(settings.executable,
         ['-extoff', url.toString()]..addAll(settings.arguments));